Influencing cellular transformation by modulating the rates of GTP hydrolysis by Cdc42

The small GTPase Cdc42 has been implicated in a number of cellular responses ranging from the regulation of the actin cytoskeletal architecture to intracellular trafficking and cell cycle progression. Cdc42 mutants that constitutively exchange GDP for GTP but still hydrolyze GTP (called 'fast-cycling' mutants) promote cellular transformation, whereas Cdc42 mutants that are unable to hydrolyze GTP and are irreversibly trapped in the GTP-bound state often inhibit cell growth. In this work, we have set out to further establish that Cdc42 needs to cycle between its 'on' and 'off' states to stimulate cell growth, by examining the consequences of manipulating its GTP-binding/GTP hydrolytic cycle in two different ways. One approach was to examine whether substitutions that act in a manner opposite to the 'fast cyclers', and extend the lifetime of the activated GTP-bound state by slowing the GTP hydrolytic reaction (i.e., 'slow-cycling' mutations), positively influence cell growth. Indeed we show that one such slow-cycling mutant, Cdc42[Y32A], which is insensitive to Cdc42GAP but still exhibits a measurable intrinsic GTP hydrolytic activity, gives rise to increased levels of activated Cdc42 in NIH 3T3 cells. We go on to show that the Y32A mutant stimulates the actin cytoskeletal changes that lead to filopodia formation, confer growth advantages to fibroblasts under low serum conditions, and enable cells to grow to high densities when exposed to normal levels of serum. The second approach was to determine whether the transforming activity of the fast-cycling Cdc42[F28L] mutant can be reversed by compensating for its accelerated nucleotide exchange reaction through the expression of the GTPase-activating protein (Cdc42GAP) and the ensuing stimulation of GTP hydrolytic activity. We showed that expression of the limit functional domain of Cdc42GAP inhibited Cdc42[F28L]-induced transformation, as well as selectively reversed the transformed phenotypes caused by the hyperactivation of wild-type Cdc42 in cells expressing the oncogenic version of Dbl (for Diffuse B cell lymphoma), a guanine nucleotide exchange factor for Cdc42 and the related Rac and Rho GTPases. Overall, the results reported here establish the requirement for Cdc42 to cycle between its signaling-on and -off states in order to positively influence cell growth and highlight how the Cdc42GAP can play an important role in regulating cell proliferation.